Electro-optical facsimile synchronizing system



April 27, 1943. w. e. H. FlNCH ELECTRO-OPTICAL FACSIMILE SYNCHRONIZINGSYSTEM 2 Sheet s-Sheet 1 Filed Oct. 17, 1940 nyvENToR. g. 71 9;141:111BY Aim! W ATTORNEY.

April 27, 1943. w, G, H, FIN H 2,317,850

ELECTED-OPTICAL FACSIMILE SYNGHRONIZING SYSTEM Filed 001:. 17, 1940 2Sheets-Sheet 2 INVENTOR Patented Apr. 27, 1943 ELECTED-OPTICAL FACSIMILESYNCHRO- NIZING SYSTEM William G. H. Finch, Newtown, Conn.

Application October 17, 1940, Serial No. 361,480

7 Claims. (Cl. 1787.1)

My invention relates in general to the art of facsimile transmission andreception and more specifically concerns a novel method for utilizin thetransmission electro-optical apparatus for the generation of distinctivesynchronizing signals.

In general, in facsimile transmission systems,

the image to be transmitted is mounted in the field of a scanning memberwhich breaks this image into elemental areas and translates the lightsand shades of each of these elemental areas into correspondingelectrical fluctuations. The scanning member may comprise conventionalmeans for producing a'concentrated spot of light upon the surface of theimage to be transmitted and gathering the light reflected therefrom andbringing it to a focus upon the sensitive surface of a photoelectriccell, as illustrated in my application Serial No. 352,938.

The system employed for breaking an image into a plurality of elementalareas comprises, as is well known in the art, a method for scanningsuccessive contiguous lines of the image. At the receiving apparatus,the electrical variations received are reconverted into lights andshades by means of a recording stylus or light beam. More specifically,the scanning process may consist of mounting the image upon a drum whichis caused to rotate while there is relative axial movement between thedrum and the scanning member. This, in effect, scans the image in acontinuous helical line.

Means must necessarily be provided for maintaining the recordingapparatus in s'ynchronism with the transmission apparatus. In presentday facsimile apparatus, synchronism is maintained by means of signalsgenerated at the termination of each scanning excursion at thetransmission apparatus and transmitted to the recording apparatus. Therecording apparatus is caused to operate at a somewhat higher speed thanthe corresponding apparatus at the transmitter and is halted at thetermination of each scanning cycle.

The initiation of the following cycle of the recording scanner iscontrolled directly by the synchronizing impulses generated at thetransmission apparatus. Thus, the recorder operates in what is termed astart-stop fashion, as determined by the receipt of synchronizingimpulses.

In present day facsimile apparatus it is conventional to employ-a camfixedly supported with respect to the drum at the transmitter which,with an associated switch, serves to generate an impulse of distinctivecharacter once during each revolution thereof. This impulse is generatedin the period during which the scanning light beam is removed from theimage itself, that is, when the scanning light beam traverses the imagemounting means.-

impulses of a distinctive amplitude or a distinctive frequency may beemployed, such as the type disclosed in my Patent No. 2,212,968. If animpulse of distinctive frequency is employed, the cam switch may beutilized to vary the frequency of a tone oscillator to that required forsynchronization and if amplitude pulses are employed, the cam switch maybe utilized to impress a signal of relatively great intensity upon thecarrier wave. At the receiving apparatus, amplitude or frequency filtersselectively operate the synchronizing mechanism and recording scanner.

The application of the cam switch to directly generate the synchronizingimpulses has often resulted in improper synchronization inasmuch asmechanical switching inherently results in switching transients andother electrical faults. The occurrence of a transient signal prior tothe actual synchronizing impulse results in a synchronizing signal inwhich there is no sharp line of demarcation to distinguish it from thecarrier wave. Thus ragged edges at the received image have been a resultof mechanical switching means for the generation of synchronizingimpulses, and have been indicative of inaccurate synchronization.

I have discovered that I may generate synchronizing impulses of eitherdistinct amplitude or frequency by means of the electro-optical scanningsystem at the transmitter, which do not present the limitationsencountered with me chanical switching, as for instance, transientoscillations. My invention contemplates the utilization of that portionof the transmitting drum which is devoted to the fastening of the image.That is, I blacken or otherwise characteristically mark the member whichfastens the image to the drum and employ this distinctive image incooperation with the electro-optical scanning system, for the generationof synchronizing impulses.

If the image mounting means comprises a flat, narrow member disposedalong the surface of the drum parallel to the axis thereof, asillustrated in my application No. 318,230, I may in one modification ofmy invention blacken this strip and arrange a synchronizing cam switchin the amplification circuit so that the level of nals of a distinctivefrequency by my novel system, I may draw a plurality of alternate blackand white parallel contiguous lines along the image fastening meansspaced so that the frequency of the light beam reflected therefrom willbe equal to the synchronizing frequency desired when the peripheralvelocity of the drum is at a predetermined value. With this type ofsynchronizing impulse, it is unnecessary to raise the level of thissignal above the normal carrier level, and accordingly, the cam and itsassociated switch will be unnecessary, and will render this.synchronizing system particularly advantageous in applications whereinswitch wear presents a practical limitation to successful operation.

It is therefore an object of my invention to provide a novel method formaintaining facsimile transmitting andreception apparatus insynchronism.

It is another object of my invention to provide electrooptical means forgenerating synchronizing signals at'facsimile transmission apparatus.

aamsoo source of motive power l3. Thus the light spot 23 traces ahelical path along the surface of the image 2.

It is desirable in present day facsimile apparatus to transmit asynchronizing impulse of the character previously described at thetermination of each revolution of the image carrying drum. This has beenaccomplished in the facsimile systems heretofore employed by means of aswitch cooperating with a cam mounted upon the drum shaft. In apreferred embodiment of my invention, I employ a cam 28 fastened to thedrum shaft l6 for actuating a switch 21 which is incorporated in thefacsimile amplifier circuit. The normal position of the switch. which isdetermined when the cam follower 3| rests upon the high or circularportion of the cam 28, is such that the facsimile amplifying apparatusis at the normal level required for signalling.

When the cam follower switch blade 3| is in the position illustrated inFigure 1, that is, in the notch 32 of the cam, the switch is oriented sothat the level of amplification is raised above It is a further objectof my invention to provide means on the field to be scanned for thegenerationof synchronizing signals of a distinctive character.

Another object of my invention is to provide means for impressingsynchronizing signals of a. distinctive amplitude or frequency upon thetransmitted carrier wave by means of the electrooptical scanning memberand associated switching means.

These and other objects of my invention will be apparent from thefollowing specification taken inconnection with the accompanyingdrawings in which:

Figure 1 is a perspective view schematically I illustrating thefacsimile apparatus of my invention: and

Figure 2 is a. schematic circuit diagram of the electrical apparatusrequired for the generation of synchronizing impulses.

The general method of facsimile transmission is schematicallyillustrated in Figure 1 wherein a drum II is utilized to carry the imageI2 to be transmitted. The drum is caused to rotate under the influenceof the source of motive power l3 through the worm l4 and the worm gearl5 which is fixedly supported upon the drum shaft IS.

The shaft It carries the drum in fixed relation therewith and isjournalled in suitable bearings to allow free rotation thereof. Scanningmeans are provided by the schematically illustrated lamp 2|, the rays ofwhich are focussed to a point 23 upon the image |2 by means of the lensor lens system 22.

Light reflected from the spot 23 on the image surface is concentratedupon the photosensitive device 24 by means of another lens or opticalsystem 25. The electrooptical system comprising lamp 2|, photoelectricalcell 24 and their associated lens systems may be combined into a unitarystructure which is displaced axially with respect to the drum by meansof a feed screw mechanism coupled bysuitable gearing to the the normallevel as will hereinafter be more completely described.

The drum II is of the type best illustrated and described in myapplication Serial No. 318,230 and is constructed so as to facilitatethe mounting and removal of the image to be transmitted. A rod 33 passesthrough the drum and is pivotable in either direction by means of handlever 34. Actuation of the hand lever 34 will cause correspondingactuation of a plurality of cranks 35 fastened to the rod 33. Thesecranks carry the guide pins 36 which in turn engage another pin 31fastened to the strip of metal 4|.

The rod 33 is normally biased so that the bar 4| is maintained incontact with the periphery of drum Rotation of the hand lever 34 in onedirection will raise a single edge of the bar 4| to permit the insertionof a corresponding edge of the image l2.

Actuation of the rod in the other direction will then lift the otheredge of the bar 4| while.

maintaining the edge of the image which had previously been fastened tothe drum in juxtaposition with the surface of the drum. This type ofconstruction greatly facilitates the mounting of the sheet 2 whichcontains the image to be transmitted.

-In one form of my invention, I blacken or otherwise distinctively colorthe outer surface of strip 4| in order to produce an impulse ofdistinctive character in the photoelectric circuit when traversed by thelight spot 23. When arranged as illustrated, in Figure l, the bar whichhas been blackened will pass under the lightbeam immediately followingthe actuation of switch 21 when cam follower 3| falls into the notch 32of cam 26, and will recede therefrom prior to the movement of switch 21to its normal position.

The application of this blackened strip to facsimile transmission willnow be considered and reference is now made to Figure 2 wherein thephotoelectric element 24 is shown with its associated amplifying andtransmission circuits. Switch 21 is also indicated with the electricalconnections thereto. The photoelectric cell cathode is joined to thenegative terminal of a battery while the anode thereof is maintained ata positive potential in relation thereto.

Thus the anode is connected to a more positive terminal of battery block42 through fixed resistor 43 and variable resistor 44. In addition, theanode of photoelectric cell 24 is also coupled directly to the controlgrid of an amplifier 45 which is herein illustrated as a pentode. Thecontrol grid bias is obtained through resistors 43 and 44 which functionas the load resistors for photoelectric cell 24 and the cathode of thepentode 45 is grounded with the suppressor grid thereof in theconventional manner. The variable resistor 44 in the plate circuit ofthe photoelectric cell permits the adjustment of the current intensityin the photoelectric circuit which corresponds with the white portion ofthe image.

The plate circuit of the pentode 45 is joined to a source of highpotential through load impedance 45 and the output voltage appearing inthe plate circuit is coupled through the coupling condenser 41 to thecentral member of switch 21.

The signal is then coupled to the triode amplifier 53 through either ofswitch members 54' or 55. and the output of amplifier 53 is coupled bymeans of coupling transformer 55 to the transmitting means which arejoined to the circuit illustrated by the terminals 51.

The control grid of amplifier 53 is negatively biased through resistorBI and is, in addition, coupled to switch member 55 so that when switchmember 5| is brought into contact with member 55 the signal coupledthrough condenser 41 is im pressed upon the control grid of amplifier53.

Three ganged. three-point switches are provided to effect control overthe facsimile transmitter circuit as shown. Thus the switches 52. 53 and64 when in position i, as indicated in Figure 2, serve to open theoutput circuit to terminals 5'! and serve to open the circuit fromterminal 65 through potentiometer 68 and coupling.

condenser 51 to the screen grid of amplifier 45.

Therefore, it will be obvious that when the three ganged switches are inposition I, no signal will appear at output terminals 51. When the threeganged switches are in position 2, the output circuit to terminals 51will be completed through switch 54 and the circuit from terminal 65 tothe screen grid of pentode 45 will be completed through couplingcondenser 52 by means of switch 52.

The position of switch 63 under these circumstances will be such thatthe output of pentode 45 may be impressed upon the control grid ofvacuum tube 53 only when the switch member 5| is in contact with switchmember 55. However, it may be seen from the corresponding numbering ofthe switch members in Figure 1 that switch members 55 and 5| will be incontact when the cam follower 3| is in the notch of cam 26 which is, aspreviously described, the period of synchronization.

During the normal period of picture transmission, when cam follower 2|is upon the raised or circular portion of cam 25, switch member 5| willcomplete the circuit through switch member 54.

transmitted signal and if oriented in position I, no signal istransmitted, in position 2, the synchronizing signals only appear at theoutput terminals; and in position 3, the picture and synchronizingsignals appear at the output. Such selective operation may be desirablein test or experimental transmissions.

When switch 21 is normally oriented, that is. switch members 5| and 54in electrical contact, the photoelectric signal is impressed upon thecontrol grid of amplifier 53 through the variable resistor II whichattenuates this signal in accordance with the value of variable resistorll, whereas the photoelectric cell signal is coupled to the control gridof amplifier 53 without attenuation when switch members 5| and 55 are incontact during the synchronizing period.

It will now be obvious that the over-all voltage amplification will begreater when switch member 5| is in contact with switch member 55.

During this period, the characteristic indication on thedrum passesunder the scanning light beam under the light spot within this time.

Pentode 45 may be employed as the modulator if the carrier frequencywhich is usually a locally generated audio frequency is impressedbetween terminal 55 and ground. The modulated carrier will then be thesignal which appears in the plate circuit of pentode 45 and upon thecontrol grid of amplifier 53.

Adjustments of potentiometer 55 will correspondingly vary the value ofcurrent which represents the black portions of the image. The functionof the variable element is therefore substantially opposite to that ofvariable resistor 44 in'the anode circuit of photosensitive element 24,which follows from the fact that the input and output voltages in avacuum tube circuit are 180 displaced in phase. It will therefore beobvious that a proper manipulation of the variable resistors 44 and 66will regulate the degree of contrast of the transmitted picture byelectrical means, thereby affording control at the transmitter of theimage to be recorded. Contrast control is particularly desirable whenrecording is performed upon electrochemically treated paper which doesnot inherently exhibit a linear characteristic.

It is to be pointed out that either a positive or a negative image maybe transmitted. Thus if a negative image is transmitted, that is, if

. the intensity of the electrical variations are opbut ganged switch 63will afford an open circuit from switch member 54. When the three gangedswitches are oriented in position 3, it may be seen that switches 62 and64 will maintain their former circuit arrangements inasmuch as switchingcontacts 2 and 3 are electrically interconnected. However, switch 53will then afford an electrical circuit from switch member 54 throughvariable resistor 1| to the control grid of amplifier 53. Thus thesignal appearing in the plate circuit of pentode 45 will be coupled tothe control grid of amplifier 55 regardless of the position ororientation of switch member 5 I. The ganged switches, therefore,exercise a control over the posite to the light variations upon theimage l2, the voltage level of the black line will be greater than thelevel of the picture signal inasmuch as resistor 'Il attenuates thepicture signal while not affecting the current which corresponds to theblack line.

These distinct amplitude impulses may be employed at the facsimilereceiving apparatus in cooperation with a start-stop system to effectproper synchronization. The increase in amplitude is obtainedimmediately after the switching operation effected by switch 21 when camfollower 3| enters the notch 32 of cam 25. The switching operationitself, however, has no effect upon the character of the synchronizingimpulse generated inasmuch as this operation merely varies theamplification of a vacuum tube circuit by the injection of resistanceprior to the actual pulse. Therefore the generation of synchronizingimpulses by means of the blackened strip 4| is not accompanied byswitching transients and other circuit irregularities.

If it is desired to employ synchronizing impulses. of a distinctivefrequency in order that selection at the receiving circuit may beeffected by means of frequency selective filters, the strip ll may becovered with a series of alternate black and white lines, the spacing ofwhich is determined by the frequency of the synchronizing impulsedesired and the peripheral velocity of the drum Ii. 4

Thus, if a synchronizing signal of 500 cycles per second is desired, anda peripheral drum velocity of 16" per second is employed, thenapproximately 31 lines per inch would be required along the face ofstrip ll in order that the frequency of the current generated within thephotoelectric cell be equalto 500 cycles per second.

The synchronizing impulse will then commence immediately as the spot 23passes over the ruled strip ll and will terminate when the "trip 4|passes beyond the spot 23. Inasmuch as increased amplitude levels arenot required when synchronizing impulses of a distinct frequency aretransmitted, the switch 21 may be dispensedwith as a means forincreasing the amplification level and the output of pentode 45 may becoupled directly to the control grid of amplifier 53 of Figure 2. 1

The aforementioned figures of 31 lines per inch is consistentwith thedefinition which is obtainable in the optical systems of facsimileapparatus. That is, it is well known that the light spot 23 may be lessthan 0.01" in diameter and therefore no difficulty is encountered inobtaining this distinctive frequency by photoelectric means.

Inasmuch as these alternate black and white lines may produce aphotoelectric cell current rich in harmonics, if solid black and whitealternate lines are used, these lines may be made to blend into eachother, the light and shades varying in a substantially sinusoidalmanner.

The receiving apparatus which may be employed with this type ofsynchronizing system is illustrated in my Patent No. 2,212,968. Hereagain the deleterious effects of switching transients do not enter toeffect improper synchronization.

Many modifications of the specific disclosures hereinabove set forth maybe made by those skilled in the art. Therefore I do not wish to belimited except by the appended claims.

I claim:

1. A facsimile transmitter comprising a rotatable drum and a picturefield mounted upon said drum, electrooptical means for helicallyscanning said picture field and deriving a current fluctuating inaccordance with the lights and shades of said field, means foramplifying said current, and means cooperating with said electro-opticalscanning means for generating an electrical synchronizing signal ofdistinctive amplitude once during each drum revolution, said lastmentioned cooperating means comprising a distinctively shaded bandacross said drum for impinging light on said electro-optical means atone end of the light intensity range to which said electro- -opticalmeans is affected in scanning said picture field, and switching meansoperative to increase the gain of said amplifier during thesynchronizing period.

2. A facsimile transmitter comprising a rotatable drum, means formounting a picture to be transmitted upon said drum, said mounting meanscomprising a blackened strip biased to juxtaposition with said drum,electrooptical means for helically scanning said picture and aarneuoderiving a current fluctuating in accordance with the lights and shadesof said picture, means for amplifying said current, said blackened stripcooperating with said scanning system once during each revolution togenerate a signal of amplitude equal to the maximum amplitude of saidpicture currents, and a switching means operative to increase the gainof said amplifier during the synchronizing period to produce asynchronizing signal which is invariably greater in amplitude than themaximum picture signal.

3. A facsimile transmitter comprising a rotatable drum, means formounting a picture to be transmitted upon said drum, said mounting meanscomprising a blackened strip biased to juxtaposition with said drum,electrooptical means for helically scanning said picture and deriving acurrent fluctuating in accordance with the lights and shades of saidpicture, means for amplifying said current, said blackened stripcooperating with said scanning system and a switching means once duringeach drum revolution to generate an electrical synchronizing signal ofan amplitude equal to the maximum amplitude of said picture currents,said switching means being in circuit with said amplifier and operativefrom a cam onsaid drum oriented with respect to said blackened strip toincrease the gain of said amplifier during the synchronizing period forproducing a synchronizing signal invariably greater in amplitude thanthe maximum picture signal.

4. A facsimile transmitter comprising a rotatable drum and a picturefield mounted upon said drum, electro-optical means for helicallyscanning said picture field and deriving a current fluctuating inaccordance with the lights and shades of said field, means foramplifying said current, and means cooperating with said scanning meansfor generating an electrical synchronizing signal of distinctivefrequency once during each drum revolution, said last mentioned meanscomprising a band of alternate black and white lines across said drum.

5. A facsimile transmitter comprising a rotatable drum, means formounting a picture to be transmitted upon said drum, said mounting meanscomprising a strip biased to juxtaposition with said drum and having aplurality of alternate black and white lines thereon, electroopticalmeans for helically scanning said picture and deriving a currentfluctuating in accordance with the lights and shades of said picture,said strip cooperating with said scanning system once during each drumrevolution to generate an electrical synchronizing signal of distinctivefrequency.

6. A facsimile transmitter comprising a rotatable drum, means formounting a picture to be transmitted upon said drum, said mounting meanscomprising a strip biased to juxtaposition with said drum and having aplurality of alternate black and white lines thereon, electroopticalmeans for helically scanning said picture and deriving a currentfluctuating in accordance with the lights and shades of said picture,said strip cooperating with said scanning system once during each drumrevolution to generate an electrical synchronizing signal of distintcivefrequency, the spacing between said black and white lines beingpredetermined to correspond with the peripheral velocity of said drumfor a given synchronizing frequency.

7. In a facsimile transmitter, a picture field, light sensitive means,scanning means for impinging light from successive points on saidpicture field on said light sensitive means in scanning cycles; meansinterposed in said picture field between said scanning cycles forimpinging light on said light sensitive means of an intensity at one ofthe limits of said image; an amplifier connected to said light sensitivemeans for amplifying currents produced thereby as said image is scanned;and switching means operated simultaneously with the impinging of lightfrom said interposed source on said light sensitive means forsimultaneously increasing the gain of said amplifier.

WILLIAM G; H. FINCH.

